For example, suppose you want to write a selector that matches the outer
selector *and* an element selector. You could write a mixin like this one that
uses the [`selector.unify()` function][] to combine `&` with a user's selector.

[`selector.unify()` function]: /documentation/modules/selector#unify

{% codeExample 'advanced-nesting' %}
  @use "sass:selector";

  @mixin unify-parent($child) {
    @at-root #{selector.unify(&, $child)} {
      @content;
    }
  }

  .wrapper .field {
    @include unify-parent("input") {
      /* ... */
    }
    @include unify-parent("select") {
      /* ... */
    }
  }
  ===
  @use "sass:selector"

  @mixin unify-parent($child)
    @at-root #{selector.unify(&, $child)}
      @content



  .wrapper .field
    @include unify-parent("input")
      /* ... */

    @include unify-parent("select")
      /* ... */
{% endcodeExample %}
